1. Field of the Invention
The present invention relates to a work brake apparatus for a wheel type excavator, and more particularly to a work brake apparatus improved for preventing the equipment from being pulled by an excavating power via keeping a suspension device locked and via applying a service brake when the wheel type excavator is at work.
2. Description of the Prior Art
Generally, a work brake apparatus for a wheel type excavator is operated by manipulating a suspension device, a service brake device, a parking brake device and a forward/backward travelling manipulation device. The respective device is installed separately as shown in FIG. 1, and the construction and the operation thereof will be described in the following.
First, the operation of the service brake device will be described. The oil discharged by a pump 2a driven by an engine 1 is accumulated in an accumulator 4 through a cut-off valve 3. If a service brake pedal 5a is pressed by a user at this time, a service brake valve 5b switches over. Then, the oil is supplied to a service brake cylinder 7 through a pilot check valve 6. Therefore, the equipment can be kept at rest.
And, when the parking brake device operates, the oil discharged by a pump 2b is pressurized by a relief valve 22. At this time, if a parking brake switch 9 is manipulated by the user, a parking brake valve 8 switches over such that the oil may apply a pilot pressure to the pilot check valve 6. Then, the pilot check valve 6 switches over to its open state such that the oil may be supplied to a parking brake cylinder 10. Therefore, the parking brake can be kept unlocked.
On the other hand, if the parking brake switch 9 is manipulated by the user, the parking brake valve 8 switches over such that the oil discharged by the pump 2b may return to a tank 25 by the relief valve 22. Then the pilot pressure applied to the pilot check valve 6 is depressurized, thus the pilot check valve 6 switches over to its closed state such that the oil supplied to the parking brake cylinder 10 may return to the tank 25 by a resilient force of a spring of the parking brake cylinder 10. Therefore, the parking brake can be kept locked.
In the work brake apparatus for a wheel type excavator, if the service brake pedal 5a is pressed by the user when the parking brake is kept locked, the service brake valve 5b switches over such that the oil may be supplied to the service brake cylinder 7 through the pilot check valve 6. Then, the service brake is applied by the pilot check valve 6. Therefore, the equipment can be prevented from being pulled by an excavating power.
Also, when the suspension device operates, if a suspension locking switch 12 is manipulated by the user, a suspension locking solenoid valve 11 switches over such that a pilot pressure to a suspension locking valve 13 may be depressurized. Then, the suspension locking valve 13 switches over as shown in FIG. 1 such that a closed circuit may be formed between the suspension locking valve 13 and a suspension locking cylinder 14. Therefore, the suspension device can be kept locked and the equipment can be prevented from shaking by an excavating power.
The state of the equipment described above is referred to as an operating state of the work brake apparatus, and is important to prevent the equipment from shaking and being pulled by an excavating power.
In a hydraulic apparatus for travelling the wheel type excavator, the oil discharged by the pump 2b is pressurized by the relief valve 22, and the oil in pressure is supplied to a forward/backward solenoid valve 16 by manipulating a travelling controller 15. At this time, the forward/backward solenoid valve 16 switches over to its neutral, forward or backward state in response to the position of the forward/backward lever 19 as shown in FIG. 1, and the oil from the forward/backward solenoid valve 16 applies a pilot pressure to a pair of ports 17a, 17b of a main control valve 17.
And, the oil discharged by a pump 2c is pressurized by a relief valve 24. If the oil is supplied to a travelling motor 18 to drive a gear box 21 in response to the state of the main control valve 17, the equipment can travel forward or backward.
However, in order to keep the work brake apparatus in the operating state when the excavator is desired to work, the user need to manipulate all the devices respectively such that the forward/backward lever may be in its neutral position, and the parking brake may be kept locked, and the service brake may be applied, and the suspension device may be kept locked. On the other hand, the user need to manipulate all the devices reversely to travel the equipment. It is complicated for the user to manipulate all the devices respectively.
And, when the equipment is at work, if the service brake device breaks and slides by an excavating power, the parking brake device is pulled and breaks, too. Moreover, if the parking brake device is included in the gear box, a failure of the parking brake device results in a successive failure of the gear box, so the equipment cannot travel. And, also in the case that the service brake is not taken off from a failure of the pilot check valve, the equipment cannot travel.